Congenital heart defects are the result of abnormal development of mesodermal cells, which form the muscular portion of the heart, or neural crest-derived cells, which populate the cardiac outflow tract and aortic arches. Defects in the two population of cells usually occur in a segmental fashion resulting in abnormalities of distinct regions of the heart with neighboring regions being relatively normal. The long term goal of this proposal is to understand the independent molecular pathways and mechanisms which could control segmental cardiac development. This type of understanding is the first step in identifying the genes which cause heart defects in distinct regions of the heart. Specifically, we focus on elucidating the pathogenesis of DiGeorge/CATCH-22 syndrome, which is a defect of cardiac and pharyngeal neural crest development, and of hypoplastic right and left ventricle syndromes. The recent discovery of the basic helix-loop-helix transcription factors, dHAND and e-HAND, provide the impetus for study in these two areas because of their expression in both cardiac mesoderm and cardiac neural crest. Furthermore, d-HAND and e-HAND, provide the impetus for study in these two areas because of their expression in both cardiac mesoderm and cardiac neural crest. Furthermore, dHAND and eHAND are expressed in unique segments of the developing heart and serve as an entrance point to dissect the upstream and downstream members of chamber-specific pathways. dHAND-null embryos have a hypoplastic right ventricle and fail to form neural crest-derived aortic arch arteries. The mechanism of dHAND action may be mediated through inhibition of programmed cell death (apoptosis) by directly regulating a potential member of the apoptotic pathway. Identification of genes downstream of dHAND have led to the discovery of cot-22, a novel gene located in the minimal DiGeorge critical region of chromosome 22 and expressed in the heart and branchial arches The three major aims of this proposal are as follows: 1) to determine, in mice and humans, if cot-22 is the gene or one of the genes responsible for DiGeorge/CATCH-22 syndrome, 2) to define the role of apoptosis in development of hypoplastic right ventricle and arch malformations in dHAND-null mice, and 3) to determine the dominant effects of dHAND and eHAND in cardiac development and post-natal cardiac disease by over-expressing the HAND genes in a cardiac and neural crest specific manner; this aim will supplement the goals of aims 1 and 2 by addressing potential mechanistic issues. The aims utilize an in vivo model which is relevant to human disease and are complemented by the studies on hypoplastic ventricles syndromes and neural crest defects proposed by other investigations in this grant. By both elucidating a molecular pathway of segmental cardiac development and positional cloning of responsible loci of specific diseases, it will be possible to identify genes responsible for distinct cardiac defects, similar to the manner in which cot-22 was discovered.